How can I prevent inserting duplicate data into a SQL Server table?

I have a series of data that need to be written into SQL, what should I do to check the data in SQL to prevent same data inserted into table?

Example data to be inserted:

David  
James  
John  

If the 4th data is John again, I want the system to skip the duplicate record (John).

So far I have:

SqlConnection myCnn = new SqlConnection(cnn);
String _state = "Insert into CamNo1(platename, date, camID, path, filename) OUTPUT INSERTED.platename values(@msg, getdate(), @camID, @path, @filename)";
SqlCommand _Query = new SqlCommand(_state, myCnn);

_Query.Parameters.AddWithValue("@msg", msg);
_Query.Parameters.AddWithValue("@camID", camID);
_Query.Parameters.AddWithValue("@path", imageFile);
_Query.Parameters.AddWithValue("@filename", name);

try
{
   myCnn.Open();
   string checkname = (string)_Query.ExecuteScalar();
   myCnn.Close();

   getcheckname = checkname;
   Console.WriteLine("OK");
}
catch (Exception)
{
}

i got the string value checkname that is last inserted, what should i do check the data?

Answer

First, you can prevent a duplicate from ever occurring in the table by using a unique index or constraint. An index/constraint can work in concert with the suggestions below. If you only use a unique index and not one of the below solutions, inserting a duplicate record will throw an error and you will need to handle that on the other end.

Additionally, I would probably insert the data via a stored procedure that checks to see if the row already exists. To do that, you can use either a MERGE statement, as shown in this pseudo code:

create procedure MyProcedure
(
    @Name nvarchar(100),
    ...
)
as

merge MyTable
using
(
    select @Name,...
) as source (Name, ...)
on MyTable.Name = source.Name
when not matched then
    insert (Name,...) values (source.Name,...)
when matched then
    update set Name = @Name,...

or, you could check for the records existence and insert or update manually:

create procedure MyProcedure
(
    @Name nvarchar(100),
    ...
)
as

    if not exists (select * from MyTable where Name = @Name)
    begin
        insert into MyTable (Name,...) values (@Name,...)
    end
    else
    begin
            update MyTable
            set ...
            where Name = @Name
    end